From '47-'53 the bellhousing crossmember mount pads were parallel to the ground, in '54 and first series '55 the crossmember was very similar except the mounts became angled - most likely to combat engine/trans movement. The bellhousing style also changed to accommodate this change.
The SM420 will bolt directly to either style bellhousing/crossmember combination and work.You should be able to remove the 3-speed torque tube trans from your '54 and install the the SM420 4-speed onto the same bellhousing using the same clutch, etc, etc. It should bolt right in.
